The overall scientific and commercial objective of the proposed research is to develop a clinical assay that can predict the recurrence of kidney stones in patients with nephrolithiasis. Since currently available diagnostic tools cannot predict who will develop recurrent kidney stones, those destined to recur would benefit from tests that could identify them in advance. We propose to develop western blotting and ELISA methods to measure various forms of a urine protein that is a known inhibitor of calcium oxalate crystallization. Preliminary studies have shown that measurement of one form of this protein combined with standard stone risk chemistries improves the sensitivity of detection of stone formers compared with stone risk chemistries alone. We will perform the assays on urine from a random sample of 50 stone formers and 50 non-stone formers. We will identify the assay(s) that best discriminate stone formers from non-stone formers, either when used alone or combined with standard stone risk chemistries. In Phase II we will prospectively test whether these measurements can identify kidney stone patients at risk for recurrence.